Method for the case carburizing of steel

ABSTRACT

In the case carburizing of low carbon (&lt;0.4%C) steels, the hardness of the case and toughness of the core are both improved by following a prescribed heat-treating sequence. After the article is carburized in contact with a carbon containing substance, it is quenched to below about 900 DEG F and maintained within the bainite region for a time sufficient to transform retained austenite. The article is then austenitized by rapidly heating to a temperature above the A3 of the core, after which it is quenched and tempered in a conventional manner. This procedure provides significant economies by permitting carburization at temperatures well in excess of 1700 DEG F, while employing high carbon potential, carburizing agents.

United States Patent 11 1 fl 11 3,891,474 Grange June 24, 1975 METHOD FOR THE CASE CARBURIZING OTHER PUBLICATIONS OF STEEL [75] Inventor: Raymond A. Grange, Washington Metals Handbook I948 683-685 p westmoreland n y. Metals Handbook, 1964, 8th 12, pages 108, 109, 111 [73] Assignee: United States Steel Corporation, & I

Pittsburgh, Pa.

Primary Examiner-C. Lovell Ffled' 1973 Attorney, Agent. or FirmArthur J. Greif [211 App]. No.: 414,545

Related US. Application Data [57] ABSTRACT In the case carburizing of low carbon 0.4%C) steels, the hardness of the case and toughness of the [52] [1.8. CI. 148/165; 148/155; 148/19, core are both improved by f n a prescribed 148/315? 148/39; 148/134? 48/143; [48/144 heat-treating sequence. After the article is carburized [51] Int. Cl C236 11/12; CZlCl 1/20 in Contact with a carbon containing substance. it is [58] Field 01 Search 148/121, 16.5, 15.5, 134, quenched to be|0w about 900 and maintained 148/143, 144, 31.5. 39, 14 within the bainite region for a time sufficient to transform retained austenite. The article is then austeni- [561 Relerences cued tized by rapidly heating to a temperature above the A UNITED STATES PATENTS of the core, after which it is quenched and tempered 1,924,099 11 1933 Bain ct a1 148/143 in a Conventional a n r. This pr cedure provides 2,128,621 8/1938 Oueneau significant economies by permitting carburization at 2,260,249 10/ 1941 Harder temperatures well in excess of 1700F, while employ- 3 3/1967 Grangc 148/143 ing high carbon potential, carburizing agents.

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METHOD FOR THE CASE CARBURIZING OF STEEL This Application is a continuation-in-part of Ser. No. 2l4,956, filed Jan. 3, I972, and now abandoned.

This invention relates to a heat treating method for improving the mechanical properties of case carburized steel articles. More specifically. the invention is related to a multi-step heat treatment which yields significant economies by eliminating the need for special alloying elements and by permitting carburization to be effected at temperatures of the order of I900F and even higher.

Case carburizing is widely employed in the production of gears, bearing races, bushings and numerous other articles in which a combination of hard surface and soft core are highly desirable. The carbon content of the surface is increased by heating a low carbon article in contact with a variety of gaseous, solid or liquid, carbon containing substances for a time sufficient to diffuse carbon to a desired case depth. After the article is carburized, various heat treatments are then employed to further harden or toughen the case only, the core only or both the case and core. Since the carburized article is in reality a duplex alloy, i.e. a high carbon case and a low carbon core, it is often very difficult to simultaneously enhance the mechanical properties of both alloy portions by the same set of heat treatments. Thus, for example, a direct quenching procedure which would yield the highest attainable core strength will yield a case with low indention hardness and unsatisfactory distortion. Similarly, a reheating treatment which may be employed to produce high surface hardness, would yield a core which is soft, but nevertheless exhibits less than maximum attainable toughness. A number of such rehardening methods, for both coarse and fine-grained steels are shown, for example, in the Metals Hundbouk, 1948 Edition, page 684.

In addition to the necessity for compromising between maximum properties for both ease and core, these prior art methods have, in general, been unsuitable for overcoming certain dificiencies which result during high temperature carburizing, i.e. at temperatures in excess of about I700F. The economic advantages of such high temperature carburizing have been long appreciated. Thus, by raising the temperature lOOF, the carburizing time for the same depth ofease, can be cut in half. Equally important, the faster diffusion of carbon at higher temperatures, advantageously yields a more gradual carbon gradient.

While a number of attempts have been made to carburize at higher than normal temperatures, these uses have been for less critical applications. Little success has been achieved in applying high temperature carburizing to more critical gear applications, since the control ofcarbon potential and case depth becomes exacting. Too high a carbon potential will result in the formation of network carbides. High temperature carburizing results in grain coarsening of both the case and core. The latter being especially important in the case of plain carbon steels (thus requiring the additional expense of grain refining elements such as Al, Ti, V, or Cb). Another factor is the increased distortion which may result during quenching.

The instant invention by employing two critically related heat treatments, overcomes these aforementioned difficulties. Subsequent to carburization at temperatures in excess of I700F, (preferably about l800-2000F) the case is first treated by the first two steps of the method of U.S. Pat. No. 3,337,376. The work piece is then rapidly austenitized to a temperature above the A of the core material for a time sufficient to substantially equalize the temperature throughout the cross-section of the core, but short enough to prevent grain growth. The piece is then tempered in a conventional manner so as to increase the yield strength and to stabilize the microstructure of both case and core, eg by heating at 350 to 450F for about 1 hour.

These and other objects and advantages of the inven tion will be better understood from the following description, taken in conjunction with the appended claims and the accompanying drawings, in which;

FIG. I is a schematic representation of the case carburizing and hardening treatment of this invention.

FIG. 2 depicts a microhardness traverse through the carburized case of 8620 steel, pack carburized for 16 hours at I700F, and

FIG. 3 is a microhardness traverse through the carbu rized case of 4310 steel, and

FIG. 4 is a microhardness traverse through the carburized case of 1382] steel.

FIG. 5 shows the effect of carburizing temperature on case depth, and

FIG. 6 shows the hardness traverses of 8620 steel, earburized at three different temperatures and conventionally hardened.

FIG. 7 shows the hardness traverse of 8620 steel carburized as in FIG. 6 and then hardened by the method of this invention.

FIG. 8 is a comparison of 4310 steel carburized at I900F for 4 hours and then hardened both conventionally and by the method of this invention, and

FIG. 9 is comparison of I3B2l steel hardened by the method of this invention and conventionally after carburization at l900F for 4 hours.

Referring to FIG. 1, the steel is earburized to increase the carbon content of the outer case to greater than 0.6%C, while the core remains that of the steel being treated (generally from O. l% to 0.3% C). The carburizing temperature may be within the range l700 to 2000F or even higher. but to achieve the economies attendant the instant process, the temperature is preferably greater than 1800F.

Following carburization, the steel is quenched (e.g. in oil) to room temperature. In the next stage, which may follow immediately, or be preceded by a desired storage at room temperature, the steel is reheated to a temperature above M but below about 950F, i.e. the temperature range in which austenite transforms to bainite. The duration at which the steel is maintained within this range is dependent on the chemical composition of the case and on the temperature selected. Ordinarily, a temperature-time combination will be employed which provides the shortest treatment time. Thus, the treatment can be selected from the finish line of the isothermal transformation diagram (see, for example, Atlas of Isothermal Transformation Diagrams, US. Steel Corporation, 1963). However, as a general rule, heating at 700F for 2 hours will be effective for almost all steel, in lieu of a treatment designed for each specific steel. As shown in FIG. I, in an alternate, preferred procedure, the steel is quenched into a salt or lead bath maintained at a temperature in the bainite region and similarly held until substantially all the austenite has transformed to bainite. This treatment is especially advantageous in treating parts in which excessive warpage or cracking might occur if quenching to room temperature were to be employed. In this regard, it should be noted that while the prior art has employed a martempering procedure for carburized parts (i.e.. quenching the steel in a salt bath maintained at a temperature above M, and then cooling in air), the tempering procedure required herein is substantially different. since it requires holding for a time sufficient to effect transformation of substantially all the retained austenite.

Following the hold in the bainite region, the workpiece may be cooled and stored at room temperature, as desired. However, it is preferred that the next stage directly follow (dashed line), so as to reduce the reheat time and heat energy required to reach peak temperature in the third step. This stage consists of a rapid reaustenitizing treatment in which the workpiece is brought to a temperature above the A of the core. This may be accomplished by induction heating, by up quenching into molten salt or lead or any other means by which the steel may be heated uniformly in a few minutes. The duration of this treatment will necessarily depend on the mass being heated and the method for accomplishing such rapid heating. For cross-sections below one square inch, a period of about one minute or less will generally suffice; however, even with significantly larger cross-sections no more than about 5 minutes should be employed. These heating times refer only to the case carburized portion which is being hardened. For example, in the case of a large gear, it is only the teeth which need be hardened, not the body of the gear.

Step IV is common to all hardening treatments of case carburized steel, and any of various tempering methods well known to the art may thus be employed.

The advantages attendant the instant invention may be seen by reference to the following Specific examples, in which three different grades of carburizing steels (8620, 4310 and 13821 were compared with controls, in which the same steel compositions were carburized and hardened by a conventional treatment (widely employed for aluminum-killed steels) which consisted of oil quenching from carburizing temperature and then tempering 1 hour at 400F. The chemical compositions of the steels tested are given in Table l, below.

cross-section of each half, starting as close to the surface as possible and continuing into the core. Hardness measurements were made with a square-base diamond indenter with a load of 500 grams, in all cases, and results are therefore expressed as Diamond Pyramid Hardness numbers (DPH). Equivalent Rockwell hardness (Re) is also reported on an auxiliary scale in all figures (See, Metals Handbook, page 98, American Soc. Metals, 1948 Edition). The results of these tests are reported in FIG. 2. The substantially higher hardness of the outer case, as well as the somewhat higher hardness of the inner case and the core is clearly evident for those samples treated by the method of this invention.

FIGS. 3 and 4 represent respectively the results of similar comparative heat treatments performed on samples of 4310 and 13B21 steels. As above, these treatments are labeled conventional" and invention." Photomicrographs were also made of each of the above samples, to study the microstructure changes responsible for the increased hardness achieved by the instant method. In addition to the increase in. hardness of the core, the core grain size was reduced from No. 8 ASTM in the conventionally hardened steels to No. l2 ASTM, in those hardened by the method of this invention. The enhanced toughness attendant such a refinement in grain size is well known to the art.

While the instant method markedly enhances the mechanical properties of both the case and the core, it is evident that the two additional steps (ll and III) will add to production costs. Therefore, the critically significant advance afforded by the instant method is that this additional cost may be offset by modifying the time consuming and highly costly carburizing treatment without suffering any diminution of these properties. High temperature carburizing is economically very attractive because, as stated hereinabove, merely raising the temperature lO0F reduces the carburizing time for the same depth by a factor of 0.5. The marked effect of such increases in carburizing temperature may be seen by reference to H0. 5, as originally shown by H. W. McQuaid, Trans, ASM, v. 25, 1937 pp. 490-519. Further economies may be realized in the instant method by employing carburizing substances (e.g. gas) with higher than normal carbon potentials. thereby achieving even further reductions in carburizing time. In many steels, it is the general practice to limit the carbon content of the outer case to a maximum of about TABLE 1 Chemical Composition Steel C Mn P S Si Ni Cr Mo Other A15! 8620 0.23 0.72 0.010 0.025 0.20 0.59 0.52 0.21 AIS! 4310 .11 .81 .001 .009 .28 1.75 .77 .22 AIS! "13821" .21 2.04 .010 .204 .25 .03 .04 .02 8-.0015

A sample of 8620 steel, 0.75 inches square by 3.5

inches long was pack carburized at 1700F for 16 hours and quenched in warm oil. One half of the bar was ad ditionally treated according to steps [1 and 111 of FIG. 1. Thus, this latter piece was heated at 700F for 2 hours in a circulating air furnace and then up quenched in lead at 1550F for 1 minute, prior to being quenched in warm oil.

Finally, both halves were tempered for 1 hour at 400F. A microhardness traverse was made on the case by inward diffusion. It will be appreciated that either of these alternatives again add to the cost of production. However, by employing the instant method, neither close control of the carbon potential nor a dif fusion treatment is necessary; and variations within the range 0.8 to I27: C in the outer case will have little or no effect on the mechanical properties of the case.

The absence in the instant method (as compared with conventional hardening) of any significant dimunition in properties, by use of high temperature carburizing, may be seen in the following examples.

Samples of the 8620 steel were pack carburized at l700F ([6 hours), l800F (8 hours) and l900F (4 hours) and hardened by oil quenching from the carburizing furnace and then tempered 1 hour at 400F (conventional"). Note that carburizing time was reduced by half for each l00F increase in temperature. This time adjustment resulted in very nearly the same case depth at l700 and l800F, but a somewhat shallower case at 1900F. This was due, in the latter instance, to the fact that about 30 minutes were required to heat the carburizing box to furnace temperature; this 30 minute heat-up time being a substantial portion of the total carburizing time for the 4-hour treatment at I900F. This could easily be overcome by increasing the carburizing time somewhat (e.g. 30 minutes) to compensate for this difference in the proportion of heat-up to hold time.

FIG. 6 shows hardness traverses of the so-treated bars. The bars carburized 1700F exhibit a case hardness which would be acceptable for most purposes, although higher hardness would be desirable. However, the bars carburized at 1800 and l900F show a very low and unsatisfactory hardness in the outer case. This undesirable condition is not completely overcome by any of the conventional hardening methods shown in the Metals Handbook, pg. 684 referred to hereinabove.

Results for comparison samples carburized at I700, l800 and I900F and treated by the method of this invention are shown in FIG. 7. Here the outer case hardness is high and substantially the same for all three carburizing temperatures. Thus, it is clearly demonstrated that the instant method permits the adoption of hightemperature carburizing without sacrifice in mechanical properties.

Similar advantages are shown for the high temperature carburizing of the 4310 and l3B2l steels as well. As before, a bar of each of these steels was pack carburized at l900F for 4 hours and oil quenched; one-half of each bar was then tempered l hour at 400F. The other half of each was treated according to the method of this invention, i.e. steps II, III, IV. FIGS. 8 and 9 respectively, show the results of hardness traverses of these samples. It is evident that the conventional treatment results in a totally unsatisfactory product. On the other hand, the ability of the inventive method to accept high-temperature carburizing in a wide variety of steels is clearly demonstrated.

I claim:

1. A method for the case carburizing ofa steel article with carbon content within the range of about 0.1 to about 0.3 percent, which comprises,

a. carburizing said article at a temperature in excess of 1800F but not greater than 2000F in contact with a carbon containing substance for a time sufficient to diffuse carbon into said article to a desired case depth. said carburization producing a maximum carbon content greater than 0.9 percent to about l.2 percent in the outer portion of the resultant carburized case,

b. quenching said article to a temperature below the M, of the case, said case still having a maximum carbon content greater than 0.9 percent,

c. reheating the article to a temperature within the bainite region for a time sufficient to transform substantially all the austenite which is retained after said quench,

cl. thereafter, reaustenitizing the lower carbon core of said article by rapidly heating to a temperature within the range A to A =l00 F of said core, and

e. maintaining the article within said A temperature range only for a time sufficient to substantially equalize the temperature throughout the crosssection of the article, and

f. immediately thereafter, cooling the article to produce a martensitic case.

2. The method of claim I, wherein said rapid austenization of the core is effected by directly heating to said A range from said bainite region.

3. The method of claim 2, wherein subsequent to step (f) the article is reheated to temper the martensitic case.

4. The method of claim 1, wherein said rapid austenitization is preceded by a quench to a temperature below the M, of said case.

5. The method of claim 4, wherein subsequent to step (f) the article is reheated to temper the martensitic case.

6. A method for the case carburizing of a steel article with a carbon content within the range of about 0.1 to about 0.3 percent, which comprises,

a. carburizing said article at a temperature in excess of 1800F but not greater than 2000F in contact with a carbon containing substance for a time sufficient to diffuse carbon into said article to a desired case depth, said carburization producing a maximum carbon content greater than 0.9 percent to about 1.2 percent in the outer portion of the resultant carburized case,

b. quenching said article to a temperature within the bainite region, said case still having a maximum carbon content greater than 0.9 percent,

c. reheating the article to a temperature within the bainite region for a time sufficient to transform substantially all the austenite which is retained after said quench,

d. thereafter, reaustenitizing the lower carbon core of said article by rapidly heating to a temperature within the range A to A +F of said core, and

e. maintaining the article within said A temperature range only for a time sufficient to substantially equalize the temperature throughout the crosssection of the article, and

f. immediately thereafter, cooling the article to produce a martensitic case.

7. The method of claim 6, wherein said rapid austenization of the core is effected by directly heating to said A range from said bainite region.

8. The method of claim 7, wherein subsequent to step (f) the article is reheated to temper the martensitic case.

9. The method of claim 6, wherein said rapid austenitization is preceded by a quench to a temperature below the M, of said case.

10. The method of claim 9, wherein subsequent to step (f) the article is reheated to temper the martensitic UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5,89l, l-7 Dated June 2 1-, 1975 Raymond A. Grange Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, paragraph (1, "A to A lOO F of said core" 0 should read A to A 100 F of said core Signed and Scaled this thirtieth Day of September 1975 [SEAL] A :resr:

RUTH C. MASON C. MARSHALL DANN :UNSIHIR ()fji'rer ('ummim'mu'r '0] Parents and Trademarks 

1. A METHOD FOR THE CASE CARBURIZING OF A STEEL ARTICLE WITH CARBON CONTENT WITHIN THE RANGE OF ABOUT 0.1 TO ABOUT 0.3 PERCENT, WHICH COMPRISES, A. CARBURIZING SAID ARTICLE AT A TEMPERATURE IN EXCESS OF 1800*F BUT NOT GREATER THAN 200*F IN CONTACT WITH A CARBON CONTAINING SUBSTANCE FOR A TIME SUFFICIENT TO DIFFUSE CARBON INTO SAID ARTICLE TO A DESIRED CASE DEPTH, SAID CARBURIZATION PRODUCING A MAXIMUM CARBON CONTENT GREATER THAN 0.9 PERCENT TO ABOUT 1.2 PERCENT IN THE OUTER PORTION OF THE RESULTANT CARBURIZED CASE. B. QUENCHING SAID ARTICLE TO A TEMPERATURE BELOW THE M8 OF THE CASE, SAID CASE STILL HAVING A MAXIMUM CARBON CONTENT GREATER THAN 0.9 PERCENT, C. REHEATING THE ARTICLE TO A TEMPERATURE WITHIN THE BAINITE REGION FOR A TIME SUFFICIENT TO TRANSFORM SUBSTANTIALLY ALL THE AUSTENITE WHICH IS RETAINED AFTER SAID QUENCH D. THEREAFTER, REAUSTENITIZING THE LOWER CARBON CORE OF SAID ARTICLE BY RAPIDLY HEATING TO A TEMPERATURE WITHIN THE RANGE A3 TO A3=100*F OF SAID CORE, AND E. MAINTAINING THE ARTICLE WITHIN SAID A3 TEMPERATURE RANGE ONLY FOR A TIME SUFFICIENT TO SUBSTANTIALLY EQUALIZE THE TEMPERATURE THROUGHOUT THE CROSS-SECTION OF THE ARTICLE, AND F. IMMEDIATELY THEREAFTER, COOLING THE ARTICLE TO PRODUCE A MARTENSITIC CASE.
 2. The method of claim 1, wherein said rapid austenization of the core is effected by directly heating to said A3 range from said bainite region.
 3. The method of claim 2, wherein subsequent to step (f) the article is reheated to temper the martensitic case.
 4. The method of claim 1, wherein said rapid austenitization is preceded by a quench to a temperature below the Ms of said case.
 5. The method of claim 4, wherein subsequent to step (f) the article is reheated to temper the martensitic case.
 6. A method for the case carburizing of a steel article with a carbon content within the range of about 0.1 to about 0.3 percent, which comprises, a. carburizing said article at a temperature in excess of 1800*F but not greater than 2000*F in contact with a carbon containing substance for a time sufficient to diffuse carbon into said article to a desired case depth, said carburization producing a maximum carbon content greater than 0.9 percent to about 1.2 percent in the outer portion of the resultant carburized case, b. quenching said article to a temperature within the bainite region, said case still having a maximum carbon content greater than 0.9 percent, c. reheating the article to a temperature within the bainite region for a time sufficient to transform substantially all the austenite which is retained after said quench, d. thereafter, reaustenitizing the lower carbon core of said article by rapidly heating to a temperature within the range A3 to A3+100*F of said core, and e. maintaining the article within said A3 temperature range only for a time sufficient to substantially equalize the temperature throughout the cross-section of the article, and f. immediately thereafter, cooling the article to produce a martensitic case.
 7. The method of claim 6, wherein said rapid austenization of the core is effected by directly heating to said A3 range from said bainite region.
 8. The method of claim 7, wherein subsequent to step (f) the article is reheated to temper the martensitic case.
 9. The method of claim 6, wherein said rapid austenitization is preceded by a quench to a temperature below the Ms of said case.
 10. The method of claim 9, wherein subsequent to step (f) the article is reheated to temper the martensitic case. 